Using ContextManager for io submodules ?

[vincentsarago]

Started in #124 (comment) I submitted the idea of using context manager for rio-tiler readers.

This is a really big change and when I started working on rio-tiler==2.0 I decided to not introduce this because I wanted to focus on other parts of the code.

Now we are close to a beta release (most of the new features have been tested and approved) It might be a good time to introduce the ContextManager form.

I've already re-written rio-tiler-crs with context manager https://github.com/cogeotiff/rio-tiler-crs#api

👍 / 👎 ?

Note: we could also let all the code as it is and just add a COGReader class !